1. Field of the Invention
The present invention is related to a gas constituent sensor for sensing the concentration of a preselected gas relative to a reference concentration and is more particularly related to an oxygen partial pressure sensor which is utilized to sense the concentration of oxygen relative to a reference gas and more particularly to indicate, through the sensing of oxygen concentration relative to a reference oxygen concentration, whether a combustible mixture was or may be combusted at stoichiometric mixture ratio.
2. Description of the Prior Art
It is known that a body of solid electrolyte, for example zirconium dioxide, which is exposed on one surface to a reference oxygen concentration and exposed on a second opposed surface to an oxygen concentration to be sensed may generate an electrical potential between the two surfaces which is indicative of the difference in oxygen concentration between the mixture being sensed and the reference mixture. Examples of such sensors may be found in U.S. Pat. Nos. 3,960,692 and 3,960,693, both issued June 1, 1976, U.S. Pat. No. 4,019,974 issued Apr. 26, 1977 and U.S. Pat. No. Re. 28,792, reissued Apr. 27, 1976, the disclosures of which are incorporated herein by reference.
By coating the surface of the zirconium dioxide, or other solid electrolyte body, which with a catalytic material, such as platinum, the sensor would generate a relatively large output signal whenever the combustion mixture is at an air/fuel ratio less than the stoichiometric mixture ratio for that fuel and will generate a relatively low signal whenever the mixture is at an air/fuel ratio greater than the stoichiometric mixture ratio for that fuel. Thus, a generally step function will be generated by the sensor, the step function going from a relatively high value to a relatively low value at stoichiometric air/fuel mixture ratio for increasing air/fuel ratios passing through stoichiometric.
Typically, as seen in the above referenced patents, the solid electrolyte is formed in the shape of a closed ended tube or a thimble, and the thimble is coated on the inside and outside with the metallic electrode, porous material, for example, platinum or palladium. The gas sensor is then inserted, particularly into the exhaust system of an internal combustion engine, whereby the exterior of the closed end tube or thimble is exposed to the heated exhaust gas created by the combustion of gases within the internal combustion engine, or is exposed to the incoming air/fuel mixture, and the interior of the close ended tube or thimble is exposed to atmospheric conditions. Thus, the sensor will generate a voltage that is proportional to the difference between the partial pressures of oxygen between the interior and exterior of the electrolyte thimble.
In order to vent the interior of the tube or thimble to atmosphere, it has been the practice to provide a vent hole either through the solid electrolyte, an end terminal, or through a protective sleeve attached to the electrolyte, the interior of the sleeve being in fluid communication with the interior of the thimble. Thus, atmospheric mixture is drawn to the interior of the sensor to provide the reference atmosphere partial pressure. Patents illustrating such sleeve venting are above referenced U.S. Pat. Nos. 3,960,692; 3,960,693 and 4,019,974.
It has been found with vent holes of this type being utilized to provide atmospheric reference oxygen to the interior of the sensor, it is possible that excess road splash, or other contaminant materials, may be drawn into the interior of the sensor through the vent and cause sensor failure due to the presence of the contaminant. Also, if sufficient contaminant exists, there may be substantial reduction in the effective area of the interior surface of the gas sensor thereby reducing the effectiveness in the sensor in producing a sufficient voltage to sense the difference in the above-noted partial pressures. A suggested solution to this problem has been to provide a silicone rubber boot over the end of the gas sensor and covering the vent hole thereby precluding splash from entering the vent hole. Such a protective boot is illustrated in above referenced U.S. Pat. No. 3,690,693. However, while the vent hole is protected, the sensor must operate in a high ambient heat environment which may cause failure of the silicon rubber boot.
It has further been found that it is desirable to provide additional positive ground to the vehicle in addition to that provided by threadably inserting the gas sensor into the exhaust system. While previous systems have provided a second positive ground, as shown in above referenced U.S. Pat. No. 4,019,974, these systems have been relatively complicated in that the separate terminal is electrically isolated from the exterior housing of the sensor. Further, separate terminals have been provided at the upper end of the gas sensor attached to a metallic portion of the sensor forming the housing. A conductor is then crimped onto the terminal, the terminal being rigidly fastened to the sensor and a leadout wire is provided for the second ground. This system is relatively complicated and expensive to that proposed by the system of the present invention.